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Oct. 30, 2008 (Washington D.C.) -- Young women who don't know they have the virus that causes genital herpes could be unknowingly fueling the herpes epidemic, new research suggests.

Although they have no symptoms of the disease and have not been tested, many women are actively shedding the virus in their genital tracts, says Kenneth Fife, MD, of Indiana University School of Medicine in Indianapolis.

"They're often infectious and don't know it," he tells WebMD.

Nationwide, at least 45 million people 12 and older, or one out of five adolescents and adults, have genital herpes, according to the CDC. It's most often caused by herpes simplex virus type 2 (HSV-2).

Fife and colleagues studied 127 young women. They entered the study when they were 14 to 18 and followed for four to six years. Only three of the women had been diagnosed with genital herpes, and the rest hadn't been tested before.

Two-Thirds of Women Actively Shedding Virus

Over the course of the study, about one-fourth of the women who originally tested negative for HSV-2 subsequently tested positive.

But what was really of concern, Fife says, was that two-thirds of the women for whom genital swabs were available were actively shedding virus. "That's when they can spread the disease," he says.

Had the women not been in the study, they might never have been tested, Fife says. Most had no symptoms -- not even common symptoms like genital pain or vaginal discharge, which can be caused by a number of disorders, he says.

The research was presented at a joint meeting of the American Society for Microbiology and the Infectious Diseases Society of America.

Viral Shedding Continues for More Than a Decade

In a separate study, University of Washington researchers found that even 10 years after being diagnosed with HSV-2, adults continue to shed virus nearly 14% of days, says Paul Auwaerter, MD, of Johns Hopkins.

The study involved 89 otherwise healthy adults with documented HSV-2 infection. As in the Indiana study, most had no symptoms, he says. Auwaerter, who served on the committee that chose which studies to highlight at the meeting, was not involved with the work.

Together, these findings "show why this virus is so successfully spread among the sexually active population," he tells WebMD.

"Many of these women had no symptoms and hence were unaware they had herpes and hence more likely to spread it. You need to be screened specifically for it," Auwaerter says.

Hormonal Contraception May Lower the Body's Natural Defenses

Also at the meeting, researchers reported that the pill and other forms of hormonal contraception may lower a woman's natural defenses against HSV-2.

Gail Shust, MD, and colleagues at Albert Einstein School of Medicine studied the genital fluid from 16 healthy women aged 18 to 25 who were at low risk of acquiring sexually transmitted diseases. Genital fluid has been shown to have natural antimicrobial activity against HSV-2, Shust says.

Nine of the 16 women were using hormonal contraception: Seven were on the pill, one used injectable Depo-Provera, and one the vaginal ring NuvaRing.

Results showed genital fluid from the women using hormonal contraception had significantly less anti-herpes activity compared with the other women, she tells WebMD.

Shust stresses that the study was small and the findings need confirmation in other research.